Thin-walled small-bore steel tube with case hardened internal surface

Abstract

The present invention relates to a process for hardening the interior surface of a long, thin-walled, small inside diameter, tubular member, such as a tube which would be used to form a downhole pump barrel. More particularly, it relates to a process involving progressive heating with an internally positioned, electro-magnetic induction coil, followed by immediate quenching with a quench ring assembly, to develop a martensitic case on the inner surface of the tube. The invention further extends to the apparatus utilized in conducting the process, and to the product tube itself.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. A long straight thin-walled small-bore steel tube 8-32 feet long, having an internal diameter of 1 1/4-3 1/4 inches and a wall 1/8-1/4 inch thick of substantially constant chemical composition, said tube wall having an internal surface case which has a substantially uniform fully transformed martensitic microstructure, which is in a state of residual compressive stress, which extends across said tube wall's entire internal surface area, and which has a substantially uniform total depth of less than about 1 mm, an effective case depth of HRC 50 of at least about 0.5 mm and a surface hardness of at least HRC 58, the remaining outer base portion of said tube wall being substantially unhardened, with a sharp demarcation between the hardened case portion and the non-hardened base portion. 
     
     
       2. The tube as set forth in claim 1 wherein: the case has an effective case thickness to HRC 50 of about 0.6-0.8 mm.   
     
     
       3. The tube as set forth in claim 2 wherein: the tube is formed of plain carbon steel.   
     
     
       4. An oil well downhole reciprocating pump barrel according to claim 1 formed of plain carbon steel. 
     
     
       5. The barrel as set forth in claim 4 wherein: the case has an effective case thickness to HRC 50 of about 0.6-0.8 mm.